General terminologies

and risk and prevention

of parasitic infections
• Amoeba: A single celled organism which has no rigid body
structure.eg include E. histolytica , Naegleria sp. and
Acanthameoba sp.

• Autoinfection: Reinfection by a parasite derived from within

the host and which is not exposed to the outside environment.
• Cilia: Small beating hairs on the outside of cells for
attachment.
• Commensal: An organism which lives within the body of another
but does not normally cause any harm
• Trophozoite, which is the motile, feeding, reproducing form
surrounded by a flexible cell membrane.
• Cyst: Resistant dormant stage of a single-celled organism which
is passed out and encourages the propagation of the species.
 Key Features of Protozoan Cysts:
Feature Description

Thick-walled, round or oval, protects

Structure
the organism inside

Survival in environment (e.g., water,

Function
soil), transmission to host

Usually in the host’s intestine before

Location Formed
being excreted

Often the infective stage (e.g., ingested

Infectivity
through contaminated food/water)
• Encystation and Excystation:
• Encystation: Transformation from trophozoite to cyst (in
response to unfavorable conditions)

• Excystation: Cyst becomes a trophozoite after entering a new

host and encountering favorable conditions (e.g., stomach
acid)
• Certain protozoa, such as Leishmania and Trypanosoma, have
flagellated forms called promastigotes or trypomastigotes
and nonflagellated forms called amastigotes.

• Definitive Host: A host in which the parasite undergoes sexual

reproduction. Fertilization occurs within the definitive host.

• Dioecious: The sexes are distinctly separate. Male

reproductive organs are present in one individual, the female
organs in another.
• Ectoparasite: A parasite which lives principally on the outer
surface of an organism.
• Endoparasite: A parasite which lives principally with the tissues
of an organism.
• Facultative parasite: An organism that may survive in the
absence of host but occasionally infects the host
• Filarial Worm: A group of long, hair-like nematodes in which
the adults live in the blood or tissues of vertebrates.
• Flagellum: A long beating hair found on a cell which normally
aids in movement.
• Flatworms: A group of organisms comprising flat bodies and
are normally hermaphroditic: refers to organisms, both plants
and animals, that possess both male and female reproductive
organs
• Flukes: A group of organisms characterized by having a flat,
unsegmented body and complex multi-stage life-cycles.

• Geohelminth: A worm which spends a certain time during its

lifecycle living in the soil.

• Host: The organism in which a parasite lives.

• Hyperinfection: Infection superimposed upon an existing

infection by the same parasite in which the parasite reaches
high numbers.
• Intermediate Host: The organism which houses the immature
or non-sexually reproducing stage of a parasite. For example,
the sheep is the normal intermediate host for the hydatid
tapeworm, while humans are the intermediate host for the
malarial parasite.

• Infestation: The term is usually applied to ectoparasites and

describes a host/parasite relationship in which the parasite lives
on the surface of the host.

• Larva: An immature stage of an organism which bears no

structural resemblance to the mature stage.
• Nematode: A group of organisms also known as the
Roundworms. Nematodes have what can only be described as
a typical "worm" shape - long, tapered at the ends and round
in cross-section.

• Obligate Parasite: A parasite which cannot survive or

reproduce outside the body of its host organism.

• Parasitism: A relationship in which an organism is wholly

dependent upon its host for food supply and provides harm to
the host.
• Pathogen: Any organism which causes harm to its host.

• Trophozoite: The active or feeding stage of a single-celled

organism.

• Vector: An organism usually an arthropod which transmits a

parasitic organism from one host to another.

• Types of Vectors:
• Biological Vector: Parasite develops or multiplies inside the
vector
• Mechanical Vector: Just carries the parasite without
development (like a contaminated syringe)

• Zoonosis: An infection of a human by an organism which is

usually parasitic in other hosts.
• MEDICAL PARASITOLOGY : is the science that deals with
organisms living in the human body (the host) and the medical
significance of this host-parasite Relationship
ASSOCIATION BETWEEN PARASITE AND HOST:
• A parasite is a living organism, which takes its nourishment and
other needs from a host; the host is an organism which
supports the parasite.
• The parasites included in medical parasitology are protozoa,
helminthes, and some arthropods.
• The hosts vary depending on whether they harbor the various
stages in parasitic development. Most of the parasites which
live in/on the body of the host do not cause disease(non-
pathogenic parasites).
• In Medical parasitology we will focus on most of the disease
causing (pathogenic) parasites
• DIFFERENT KINDS OF HOSTS
• • Definitive host – a host that harbors a parasite in the adult
stage or where the parasite undergoes a sexual method of
reproduction.

• • Intermediate host - harbors the larval stages of the parasite

or an asexual cycle of development takes place. In some cases,
larval development is completed in two different intermediate
hosts, referred to as first and second intermediate hosts.
• Paratenic host – a host that serves as a temporary refuge and
vehicle for reaching an obligatory host, usually the definitive
host, i.e. it is not necessary for the completion of the parasites
life cycle.

• Reservoir host – a host that makes the parasite available for

the transmission to another host and is usually not affected by
the infection.

• Natural host – a host that is naturally infected with certain

species of parasite.

• Accidental host – a host that is under normal circumstances

not infected with the parasite
• DIFFERENT KINDS OF PARASITES
• • Ectoparasite – a parasitic organism that lives on the outer
surface of its host, e.g. lice, ticks, mites etc.

• • Endoparasites – parasites that live inside the body of their

host, e.g. Entamoeba histolytica.

• • Obligate Parasite - This parasite is completely dependent on

the host during a segment or all of its life cycle, e.g. Plasmodium
spp.
• Facultative parasite – an organism that exhibits both parasitic
and non- parasitic modes of living and hence does not
absolutely depend on the parasitic way of life, but is capable
of adapting to it if placed on a host. E.g. Naegleria fowleri

• Accidental parasite – when a parasite attacks an unnatural

host and survives. e.g.Hymenolepis diminuta (rat tapeworm).

• Erratic parasite - is one that wanders in to an organ in which it

is not usually found. e.g. Entamoeba histolytica in the liver or
lung of humans.
• Symbiosis
• There is a dynamic equilibrium which exists in the interaction
of organisms. Any organism that spends a portion or all of its
life cycle intimately associated with another organism of a
different species is considered as Symbiont (symbiote) and this
relationship is called symbiosis (symbiotic relationships). The
following are the three common symbiotic relationships
between two organisms:
Mutualism –
• an association in which both partners are metabolically
dependent upon each other and one cannot live without the
help of the other; however, none of the partners suffers any
harm from the association.
Commensalism
• an association in which the commensal takes the benefit
without causing injury to the host. E.g. Most of the normal
floras of the humans’ body can be considered as commensals.
Parasitism
• An association where one of the partners is harmed and the
other lives at the expense of the other. E.g. Worms like Ascaris
lumbricoides reside in the gastrointestinal tract of man, and
feed on important items of intestinal food causing various
illnesses
Risk Factors for Parasitic
Infections
• Poor hygiene and sanitation
→ Can lead to fecal-oral infections like Ascaris lumbricoides.
• Contaminated food and water
→ Drinking untreated water may cause Giardia lamblia
infection.
• Eating undercooked meat
→ Pork containing cysts of Taenia solium may result in
neurocysticercosis.
• Walking barefoot in contaminated soil
→ Skin penetration by Hookworm larvae such as Ancylostoma
duodenale.
• Close contact with pets or stray animals
→ Children may acquire Toxocara canis, causing visceral larva
migrans.

• Travel to endemic areas

→ Exposure to malaria parasites like Plasmodium falciparum.

• Immunocompromised states
→ Use of steroids or organ transplant can cause Strongyloides
hyperinfection.
 Prevention of Parasitic
Infections
• Maintain good hand hygiene
→ Wash hands after toilet use and before eating to avoid oral
transmission.
• Use safe drinking water
→ Boil or filter water to kill protozoan cysts like
Cryptosporidium.
• Cook meat thoroughly
→ Prevents infections by Toxoplasma gondii and tapeworms.
• Wear protective footwear
→ Avoid skin exposure to larvae in soil (e.g., Hookworm).
• Use insect repellents and mosquito nets
→ Essential to prevent vector-borne diseases like malaria and
leishmaniasis.

• Avoid swimming in freshwater in endemic regions

→ Prevents infection with Schistosoma.

• Deworming programs
→ Regular treatment helps reduce soil-transmitted helminths,
especially in children.
Route of Transmission

• The route of transmission refers to the pathway by which a

parasite (or any infectious agent) enters the human body and
causes infection. Understanding these routes is crucial for
preventing and controlling parasitic diseases.
• Types of Transmission
• 1. Direct Transmission
• The parasite is transferred immediately from one host to
another without an intermediate object or vector.
• Examples:
• Person-to-person contact:
• Enterobius vermicularis (pinworm) – spread by hand-to-mouth
contact after scratching
• Trichomonas vaginalis – sexually transmitted
• Autoinfection:
• Strongyloides stercoralis – internal cycle reinfects the same host
• Taenia solium – ingestion of own eggs causing cysticercosis
• 2. Indirect Transmission
• The parasite is transferred through an intermediate source,
such as contaminated food, water, soil, or a vector.
Main routes of transmission of
parasites
• 1. Fecal-Oral Route (Ingestion of Contaminated Food/Water)
• Parasites enter the host through ingestion of contaminated
water, food, or hands.
• Examples:
• Entamoeba histolytica – contaminated water or vegetables
• Giardia lamblia – drinking water from streams
• Ascaris lumbricoides – ingestion of soil-contaminated food
• Taenia solium – eating undercooked pork
• 2. Skin Penetration
• Larvae penetrate the skin directly, often from soil or water.
• Examples:
• Ancylostoma duodenale (hookworm) – barefoot contact with
soil
• Strongyloides stercoralis – skin contact with contaminated soil
• Schistosoma spp. – penetration through skin while swimming
• Vector-Borne Transmission
• Parasites are transmitted by insect vectors through bites.
• Examples:
• Plasmodium spp. (malaria) – via female Anopheles mosquito
• Leishmania spp. – via sandfly
• Trypanosoma cruzi – via triatomine (kissing) bug
• Wuchereria bancrofti – via mosquito bite
• 4. Autoinfection
• Reinfection by the same parasite within the same host
without external exposure.
• Examples:
• Strongyloides stercoralis – internal cycle leads to
hyperinfection
• Taenia solium – ingestion of eggs causes cysticercosis
• Enterobius vermicularis – scratching perianal area and
swallowing eggs
• 5. Person-to-Person (Direct Contact)
• Close physical contact or through shared contaminated items.
• Examples:
• Enterobius vermicularis – eggs spread through hands,
bedding, clothing
• Trichomonas vaginalis – sexual transmission
• 6. Zoonotic Transmission (Animal to Human)
• Humans get infected from animals directly or via
contaminated environment.
• Examples:
• Toxocara canis – from dog feces (visceral larva migrans)
• Echinococcus granulosus – from dogs (hydatid disease)
• Cryptosporidium – from cattle or petting zoos